# [R] [OOPS] Re: Drawing sample from a circle

(Ted Harding) Ted.Harding at manchester.ac.uk
Mon Jun 21 15:07:26 CEST 2010

```I think you have misunderstood the code! In case [B] below (sampling
uniformly over the area within the circle) the assignment

r <- sqrt(runif(n))

does just that: The radius distribution (of r) is not uniform;
the distribution of r^2 is uniform, and r^2 "is proportional to the
circle area".

In other words: Given a unit circle (0 < r < 1), the probability
that r falls within 0 < r < r0 [ < 1 ] is

Prob(r < r0) = prob(r^2 < r0^2) = r0^2

since r0^2 is uniformly distributed on (0,1). And the area of
the circle r < r0 is pi*r0^2. Along with the uniform distribution
of the angle u on (0,2*pi), this then gives points uniformly
distributed over the area within the circle r < 1.

Ted.

On 21-Jun-10 12:45:42, R Heberto Ghezzo, Dr wrote:
> Sorry Ted
> The code for the circle is in error, the radius distribution should be
> proportional to the circle area not
> uniform. Better / simple to sample from the (-1,1) square uniformly and
> reject where x^2+y^2 > 1
> Heberto
> ________________________________________
> From: r-help-bounces at r-project.org [r-help-bounces at r-project.org] On
> Behalf Of Ted Harding [Ted.Harding at manchester.ac.uk]
> Sent: Friday, June 18, 2010 5:48 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] [OOPS] Re:  Drawing sample from a circle
>
> OOPS: AN error on the code below! See in-line.
> Ted.
>
> On 18-Jun-10 09:33:04, Ted Harding wrote:
>> On 18-Jun-10 08:04:36, Ron Michael wrote:
>>> Hi, I would like to draw 10 uniformly distributed sample points from
>>> a
>>> circle with redius one and centered at (0,0). Is there any R function
>>> to do that?
>>> _
>>> Thanks,
>>
>> You can quite easily write one.
>>
>> [A]
>> Sampling uniformly on the circumference of the circle:
>>
>>     x0 <- centre[1] ; y0 <- centre[2]
>>     u <- 2*pi*runif(n)
>>   }
>>
>># Returns an nx2 matrix whose two columns are the x and y coordinates
> CORRECTION:
>
>     x0 <- centre[1] ; y0 <- centre[2]
>     u <- 2*pi*runif(n)
>   }
>
># Returns an nx2 matrix whose two columns are the x and y coordinates
>
>> [B]
>> Sampling uniformaly within the circle
>>
>>     x0 <- centre[1] ; y0 <- centre[2]
>>     u <- 2*pi*runif(n)
>>     r <- sqrt(runif(n))
>> }
>>
>># Returns an nx2 matrix whose two columns are the x and y coordinates
> CORRECTION:
>
>     x0 <- centre[1] ; y0 <- centre[2]
>     u <- 2*pi*runif(n)
>     r <- sqrt(runif(n))
> }
>
># Returns an nx2 matrix whose two columns are the x and y coordinates
>
>
>> [C]
>> Examples:
>>
>>   plot(csamp(100),asp=1)
>>
>>   plot(Csamp(1000),asp=1)
>>
>> Ted.
>>
>> --------------------------------------------------------------------
>> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
>> Fax-to-email: +44 (0)870 094 0861
>> Date: 18-Jun-10                                       Time: 10:33:00
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>
> --------------------------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
> Fax-to-email: +44 (0)870 094 0861
> Date: 18-Jun-10                                       Time: 10:48:14
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